Satellite communication systems relay signals between user ground terminals and gateway ground stations over channels consisting of a range of frequencies. For most coverage areas, the available frequency spectrum or bandwidth for uplink and downlink satellite transmissions is a limited resource. The growing demand for satellite communications has thus increased the need for higher levels of bandwidth efficiency, which can provide the benefits of increasing capacity in a given bandwidth and reducing pricing to satellite users.
Uplink and downlink transmissions between satellites and terrestrial stations may occur through the use of multiple regional beams and/or spot beams. Examples of beam coverage areas are shown in FIGS. 9 and 10, which illustrate examples of spot beams and regional beams, respectively. Regional beams are wide beams that serve a broader area and do not require precise directionality to receiving dishes. For example, a regional beam might serve the continental United States. A spot beam, in contrast, is spectrally concentrated in power to cover a specific, limited geographic area. Spot beams may have radii of, for example, 300 or 500 miles. Often, neighboring spot beams are clustered to serve a larger geographic area where the size of the spot beam may be determined by factors such as a threshold for loss. For example, the radius of the spot beams may be sized according to locations that experience a 3 dB to 7 dB loss relative to the central portion of the beam that experiences maximum gain.
Neighboring spot beams suffer from mutual interference near their shared boundaries, and this interference can compound problems relating to limited bandwidth in satellite transmissions because the interference constrains choice of bandwidth in one spot beam based on its neighboring spot beam. Methods of frequency assignment that involve reusing and/or sharing a portion of the frequency spectrum between satellite spot beams have been proposed to address the problem of interference and accompanying inefficiencies in bandwidth usage.
In addition, satellite communications require transmissions over designated uplink and downlink channels. Each satellite channel is a spectrum of frequencies. Commonly used frequency bands for satellite communications include L-band (1-2 GHz), S-band (2-4 GHz), C-band (4-8 GHz), Ku-band (12-18 Ghz), and Ka-band (18-27 GHz). Often, the lower portion of each band is dedicated to downlink channels, wherein the transmission is sent from a satellite to the ground, while the upper portion of each band is dedicated to uplink channels, wherein the transmission is sent from terrestrial stations to the satellite. To transmit data within a band, source signals are combined with carrier waves whose frequency is within the band and, more particularly, within a channel.
One method of increasing bandwidth efficiency in satellite communications is to modulate individual source signals prior to transmission. For example, Digital Video Broadcasting (DVB) is a family of standards for modulating source signals in satellite broadcasting. DVB standards include the DVB-S2 and DVB-S2X standards, which provide data framing structures, channel coding, and modulation for combining individual source signals with common carrier transmit signals. DVB standards may involve MPEG compression of data signals for more efficient bandwidth use.
An additional approach to increasing bandwidth efficiency that may be used in combination with modulation methods like DVB-S2 is to combine source signals during transmission. DVB standards are designed to carry single or multiple compressed MPEG streams (signals) in a single satellite transmission, i.e., to transmit multiple source signals on overlapping channels, which results in more efficient use of the available bandwidth.
A number of methods for modulating signals in satellite communications networks are known in the field. For example, signals may be transmitted using Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and/or other modulation methods. In FDMA, all users share a channel comprising a frequency spectrum, but each transmits at a specific subset of the frequency spectrum within a channel. In TDMA, multiple users share the same channel by transmitting in short bursts alternating with other users. In an example variant of FDMA, Orthogonal Frequency-Division Multiplexing (OFDM), closely neighboring channels and sub-channels are polarized to be orthogonal to one another, decreasing signal interference. Amplitude Phase Shift Keying (APSK) is another example modulation method, in which the phase and amplitude of a carrier wave are modulated between a finite set of specific amplitudes and phase shifts to transmit information in a source signal.
Traditional two-carrier channel sharing or carrier-in-carrier channel sharing represents another approach to increasing bandwidth or spectral efficiency, wherein a shared, overlapping bandwidth is used for uplink and downlink transmissions. Cancellation at both ends of a communication link involves providing an estimate or copy of the undesired transmitted signal to extract the desired received signal from the received combined signal.
The above-mentioned techniques suffer from a number of drawbacks, including limitations on bandwidth efficiency and combining multiple signals into overlapping channels due to interference. For example, TDMA and other such methods do not involve overlapping channels. Also, channels in FDMA are not overlapping but adjacent, as an overlap using an FDMA-based technique can give rise to signal interference. Furthermore, approaches that involve overlapping signals, like carrier-in-carrier channel sharing, require access to the original source signals to cancel undesired signals and accurately extract desired signals. This typically limits the overlapping signals to share the same uplink and downlink beam.